The invention relates to a method and a device for coating substrates in a vacuum in which a plasma is generated from a target using a laser beam and ionized particles of the plasma are deposited on the substrate in the form of a layer.
With a very wide range of known PVD methods which can be used to provide a very wide range of substrates with different coatings, it is known to influence certain properties of the layer by the addition of various gases or gas mixtures. Such gases or gas mixtures are added to the vacuum chamber at a correspondingly adjustable gas pressure. In a large number of these known methods, a certain proportion of a gas is at the same time sucked out again through a corresponding outlet opening.
On account of chemical reactions which take place when a reactive gas or gas mixture is supplied, the compounds which are formed with the vaporized target material and the corresponding gas component can be deposited as a layer, such as for example an oxide or nitride layer. The chemical reactions which take place in the vacuum chamber can only be influenced to a limited extent, and it is not generally possible to preclude inhomogeneities which have an adverse effect on the desired properties.
In a very wide range of PVD methods, a plasma is generated from a target and the ionized particles from the plasma are able to form a layer on the surface of a substrate. The plasma can be generated in a wide variety of ways, the solution according to the invention being particularly advantageously aimed at those PVb methods which are carried out the generation the plasma using a laser beam which is directed onto the target. The plasma can be generated purely with the aid of a laser beam of sufficient intensity or in combination with a vacuum arc discharge, which may also be ignited with laser assistance. Known methods of this nanotubes or fullerenes can be obtained in a layer of this type. Such structures may be incorporated in an amorphous layer in the form of powders or groups of fibers.
However, with these known solutions it is only possible to achieve relatively low yields or coating rates and to coat areas of relatively small dimensions. Moreover, the desired crystalline carbon structures are incorporated randomly with a low concentration in a layer of this type, and there is no possibility of having a controlled influence on a desired orientation or texturing.
Principal reasons for these drawbacks are that, on account of the high particle pressure in the plasma, it is substantially impossible for gases to penetrate into the plasma.
Moreover, a method for producing multicomponent materials in which a substrate is to be coated with ultrafine particles is known from U.S. Pat. No. 5,254,832 and EP 0 437 890 A1. The ultrafine particles are applied together with a further material component; this component can be applied by reactive deposition from an additional gas phase, by means of a pure CVD method or a correspondingly modified CVD method. In this case, an inert carrier gas is also used, which in one alternative is directed onto the surface of the target and, from there, toward the substrate and, in a second alternative, is directed onto the substrate through a target, which has been drilled through by a laser, and an opening in a diaphragm.
Therefore, it is an object of the invention, starting from known methods for coating substrates in a vacuum in which a plasma is generated from a target, to allow gases or gas mixtures to be supplied in a locally and/or temporally defined manner.
According to the invention, this object is achieved by the features of claims 1 and 4 for method and the features of claim 13 for a device for carrying out a method of this type. Advantageous configurations and refinements of the invention will emerge when the features given in the dependent claims are used. According to the invention, it is intended, starting from the methods described in DE 39 01 401 C1, DD 279 695 B5, CD 280 338 B5 and U.S. Pat. No. 4,987,007, gases or gas mixtures to be supplied in a locally and/or temporally defined manner.
According to the invention, this object is preferably achieved by the characterizing features of the invention. Advantageous embodiments and further developments will be apparent from the description of the invention provided herein. According to the invention, it is intended, starting from the methods described in DE 39 01 401 C1, DD 279 695 B5, DD 280 338 B5 and U.S. Pat. No. 4,987,007, to the full disclosure content of each of which reference is to be made for the invention, to provide the supplementary measure that at least one of the targets used has a certain degree of porosity which is suitable for fulfilling a temporary storage function for a gas or gas mixture in sufficient quantity and is suitable, both alone or in combination with the temporary storage function, for supplying the gas to the plasma through the pores in a temporally and/or locally defined manner.
It is advantageous to use a pulsed laser beam if the plasma, irrespective of whether this is generated by means of laser beam alone or by the combined use of laser beam and vacuum arc discharge, it being possible to appropriately select a suitable laser light source for the various possible target materials, in order to achieve a high level of efficiency.
In the laser arc process, the target should consist of an electrically conductive material, such as for example C, Al, Cu, Fe, Si, Ti another metal, alloys or chemical compounds. However, it is also possible to use other elements or their compounds, for example B or Ge. To ensure the required porosity, it is advantageously possible to use sintered bodies and/or fiber-reinforced composites for this purpose.
If the plasma is generated by means of laser beam alone, it is also possible to use targets made from electrically nonconductive materials, such as for example a very wide variety of oxides, nitrides and carbides.
As has already been mentioned, a target which is designed and is to be used in accordance with the invention can serve as a temporary store for the various gases or gas mixtures to be used. In this case, infiltration of the target with the corresponding gas or gas mixture, preferably up to its saturation limit, is carried out prior to the actual initiation of coating, and then the actual coating operation is initiated.
However, the corresponding gas or gas mixture may also. be supplied during the coating or, if the storage capacity of a target is inadequate, may be added during the coating via cavities formed inside the target or via a preferably tubular gas supply through the pores. However, the procedure should be such that the pores of the target which have emptied as a result of gas being consumed are refilled, so that the temporary storage function of the target is maintained.
The gas used may, for example, be inert gases (e.g. the gases of main group VIII), O2, N2, H2 or hydrocarbon compounds or gas mixtures thereof, but also other gases and gas mixtures.
Apertures may preferably be formed in such a gas supply, through which apertures, at a constant gas pressure, a metered supply of gas to the plasma can be achieved on account of the restricting action of the apertures. This results in a further advantage, in that the corresponding gas or gas mixture can be passed via a circuit, of which the gas supply forms part, and the target is cooled by the gas, which is circulated in excess.
With the solution according to the invention it is particularly advantageously possible to produce diamond-like carbon coatings in which crystalline nanostructures are formed in a homogenously distributed manner. In this case, the possibility of both temporally and locally defined supply of gas has a particularly beneficial effect during the plasma generation.
Moreover, coordinated movement of the substrate (in translation, rotational pivoting/turning about an axis or a point) with respect to the plasma may lead to controlled texturing, i.e. a designed orientation of the nanocrystals.
The targets which are to be used in the invention should have a porosity of at least 3%, preferably more than 5%, in order to fulfill the desired properties.
If, by way of example, a multilayer structure of various layers is to be formed, it is possible to use at least two different suitable target materials. They may be arranged separately from one another, in which case at least one of the different targets is modified in accordance with the invention. It is also possible to supply different gases or gas mixtures over or out of these targets.
The various target materials may also be arranged immediately adjacent to one another, as may be the case, for example, when they form partial disks of a target in the form of a roller. In the latter case, the individual partial disks may be connected to separate gas supplies and, if appropriate, outlets.